Governments have been imposing progressive mandates for reducing amounts of particulate matter (PM) in exhaust emissions. The diesel particulate matter filter (DPF) has been developed for exhaust aftertreatment systems to remove diesel particulate matter containing soot, unburned fuel, lubrication oil etc. from the exhaust gas.
A DPF typically includes a filter encased in a canister that is positioned in the diesel exhaust stream. The filter is designed to collect PM while allowing exhaust gases to pass through it. Types of DPFs include ceramic and silicon carbide materials, fiber wound cartridges, knitted fiber silica coils, wire mesh and sintered metals. DPFs have demonstrated reductions in PM by up to 90% or more, and can be used together with a DOC to reduce HC, CO, and soluble organic fraction (SOF) of PM in diesel exhaust.
Because a DPF traps soot and other PM, it must be regenerated from time-to-time because the volume of PM generated by a diesel engine is sufficient to fill up and plug a DPF in a relatively short time. The regeneration process burns off or “oxidizes” PM that has accumulated in the filter. However, because diesel exhaust temperatures often are not sufficiently high to burn accumulated PM, various ways to raise the exhaust gas temperature or to lower the oxidation temperature are utilized. Regeneration can be accomplished passively by adding a catalyst to the filter. For example, a diesel oxidation catalyst (DOC) can be provided upstream of a DPF to oxidize NO to generate NO2 (requiring accurate control to maintain the mass ratio of NO/PM in engine-out exhaust gas), which in turn oxidizes the PM in the downstream DPF. Alternatively, regeneration can be achieved actively by increasing the exhaust temperature through a variety of approaches, a fuel burner, resistive heating coils or late fuel injection.
However, running active DPF regeneration cycles involve injecting energy into the engine system and result in excess fuel use, and thus excess cost. Further, managing the soot load in the DPF in lean burning engine systems to reduce active cycling is difficult because operation of the engine system frequently involves soot producing transients. For instance, transients involving throttling increase fuel amounts in the air to fuel mixture such that the ratio can approach or exceed stoichiometric levels, resulting in excessive PM trapped by the DPF, and consequently requiring more active regeneration cycles.